Add wspd and wdir as separate diagnostics variables

[hugohartmann]

Currently wrf-python provides wspd_wdir as a combination of wind speed and direction. Can you please add wind speed and direction as individual parameters (e.g. wspd and wdir)?

[bladwig1]

@hugohartmann Is this causing to a particular problem or is this just an annoyance?

[hugohartmann]

I am using GIS4WRF, and I understood from the developers (@letmaik) that they only support single-quantity variables. As they make use of the wrf-python package, the suggested me to ask you to add wind speed and direction as separate parameters, for both the 2D U10,V10 fields and the 3D U,V fields (if possible).